For smooth animation, you probably need more than just the "mouth open" and "mouth closed" images. You need two or three intermediate images.

To make resource management easier, you might want to put all of your animation frames together in a single, wide image, that will look like a "filmstrip". Then, to draw a frame, you use the (x, y) offset for the frame your interested in.

Finally, if you draw all the frames each time you go through the main loop of your program, Pac-Man will do a complete chomp every time you move him. You should think about drawing just one frame each time through the main loop and using a variable to track which frame you're on.